Non-asbestos flexible sheet material

ABSTRACT

PCT No. PCT/GB92/00520 Sec. 371 Date May 24, 1994 Sec. 102(e) Date May 24, 1994 PCT Filed Mar. 23, 1992 PCT Pub. No. WO92/17418 PCT Pub. Date Oct. 15, 1992.Non-asbestos flexible sheet material suitable for use in gaskets (e.g., automobile exhaust manifold gaskets) operating at high temperature (e.g., 500 DEG -700 DEG  C.) is made by impregnating a non-woven tissue of glass or like fiber stable at 700 DEG  C. with an aqueous slurry of inorganic filler, particularly a clay, said aqueous slurry including a dispersed elastomeric binder (e.g., an acrylic) and a dispersed or dissolved binder (e.g., colloidal silica) which is essentially inorganic and is effective above 300 DEG  C. The impregnated tissue is then dried.

FIELD OF THE INVENTION

This invention relates to non-asbestos flexible sheet material suitablefor use in gaskets operating at high temperature, such as those for theexhaust manifold of an internal combustion engine.

DESCRIPTION OF THE PRIOR ART

Flexible sheet material having a basis of asbestos is made of fibers ofasbestos bound together with a binder (often starch) and is made withthe use of conventional paper-making machinery such as a Fourdriniermachine. In the manufacture of exhaust manifold gaskets, asbestos paperis drawn from a reel and, with a sheet of the metal (e.g. tinplate)which is to form the support element of the gasket, is passed betweenrollers which urge the two sheets into engagement with one another, asby pushing shallow tangs on the metal sheet into the paper.Additionally, before passage of the sheets between the rollers, adhesivemay be applied to the metal sheet or to the paper to improve bondingafter engagement. From the composite sheet thus formed blanks are cut inthe desired shape. Composite sheet as just described is usually made inthree forms: single-sided (one layer of metal support, and one layer ofpaper); double-sided paper; and double-sided metal. The paper employedis ordinarily made so that it will serve for all three forms of gasket.

SUMMARY OF THE INVENTION

The present invention is concerned to provide flexible sheet materialnot having a basis of asbestos, but with the properties required for usein exhaust manifold and like gaskets, and in particular the flexibilityand tensile strength to withstand being made up into gaskets withoutlosing its integrity, the ability to withstand temperatures of 500°-700°C., and the ability to retain its sealing capacity despite the constantfluctuations in temperature to which it may be subjected in use.

It is already known, e.g. from our GB 2 138 854 A, to make suchnon-asbestos flexible sheet material by a paper-making procedure. We nowprovide an alternative process, which utilizes non-woven web of glass(such as is now commercially available for the manufacture ofglass-reinforced plastic articles) or like fiber stable at 700° C.

The method of the invention can be employed to produce composite sheethaving a sealing performance superior to that of sheet utilizing anon-asbestos paper, as in its better stress-retention under clampingbolt pressure or its improved resistance to loss of integrity duringprolonged use.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides a method of making non-asbestos flexible sheetmaterial suitable for use in gaskets operating at high temperature, inwhich a non-woven web of inorganic fibers stable at 700° C. isimpregnated with an aqueous slurry of inorganic filler, said aqueousslurry including a dispersed elastomeric binder and a dispersed ordissolved binder which is essentially inorganic and is effective above300° C.; and the impregnated web is then dried.

The inorganic fibre may be mineral or carbon fiber, but is preferablyglass fiber.

The inorganic filler is the major weight component of the slurry, andmay be chosen from materials commonly used in papermaking e.g. barytes,mica, talc, limestone. A mixture of materials may be used. Preferablythe inorganic filler is predominantly a clay, such as ball clay, chinaclay or calcined china clay, for clays help to confer a desirablethixotropy which makes for stability and homogeneity in the slurry, withthe suspended solids showing little tendency to settle out, and enablesa smooth surface finish to be readily obtained on the impregnated web.

The elastomeric binder which is dispersed in the aqueous slurry providesflexibility and compressibility in the sheet material produced, and actsas a binder at temperatures up to about 300° C. It is, of course, aningredient which is useful in the manufacture stage but quickly becomesdestroyed when the gasket product is used at temperatures of 500°-700°C. Suitable elastomers are of the acrylic or nitrile butadiene rubbertype, but any elastomer is conveniently used which is available in latexform.

The inorganic binder which is dispersed or dissolved in the aqueousslurry maintains the integrity of the material after the elastomericbinder has burnt off when the gasket product is exposed to operatingtemperatures. It is preferably a silica-type binder, such as colloidalsilica or tetraethoxysilane, Si(OC₂ H₅)₄, whose ethoxy groups readilyhydrolyse off and leave silica as the essential binding component. Analkali metal silicate, suitably sodium silicate, may also be employed asinorganic binder.

Preferred dry weight proportions of the ingredients of the aqueousimpregnating slurry are:

    ______________________________________                                                         % by weight                                                  ______________________________________                                        Inorganic filler   75-95                                                      Inorganic binder    1-15                                                      Elastomeric binder  1-12, especially 1-10                                     ______________________________________                                    

The invention further illustrated by the following Examples.

EXAMPLE 1

This example describes the manufacture of non-asbestos flexible sheetmaterial from a commercially available non-woven glass fiber web and anaqueous slurry of the composition

    ______________________________________                                                           % by dry weight                                            ______________________________________                                        Mineral filler (calcined china clay)                                                               88.43                                                    Inorganic binder (silica type)                                                                     5.54                                                     Elastomeric binder (an acrylic)                                                                    5.43                                                     Pigment (carbon black)                                                                             0.60                                                     ______________________________________                                    

An aqueous colloidal dispersion of silica (Nalfloc 1030, 3.3 kg; 30% byweight SiO₂ ; pH 9.9; Na₂ O content, 0.5%) was added to water (5.82 kg)in a vessel equipped with a stirrer, and a dispersing agent (Dispex® N40of Allied Colloids; a sodium polycarboxylate; pH 7-7.5; 5 ml, activecontent 40% by weight) was added to the mixture. Calcined china clay (15kg; at least 50% by weight of particle size below 2 μm; specific surfacearea, 12 m² /gram; moisture content, 0.5% max) was then stirred in overa period of 20 minutes, with increase of stirrer speed as the slurryformed on addition became more viscous. To impart thixotropy aqueoussodium carbonate solution (10% by weight Na₂ CO₃ ; 1.58 kg) was added tothe slurry followed by a further quantity of calcined china clay (0.8kg).

An elastomeric binder was incorporated in the slurry by adding to it anaqueous dispersion (2.18 kg) of an acrylic rubber in the form of acommercially available latex (Revacryl 272; 45.5% solids content), beingan emulsion of a self cross-linking acrylic copolymer; pH 4; particlesize 0.2 μm; glass transition temperature Tg of dry film, -44° C.

An aqueous carbon black dispersion (Croda Forthsperse® black; 0.36 kg;30% solids) was added, and the mixture was given a final 5 minutesstirring at high speed to ensure homogeneity.

The slurry resulting from the above procedure was thixotropic, and wasemployed as impregnant for a commercially available non-woven glassfiber web (weight per unit area=100 grams/m²) made by a wet layingprocess from E-glass fibers of length 18 mm and diameter 13 μm, thebonding agent being a urea formaldehyde resin, forming about 10% byweight of the web.

The glass web was unwound from a roll and passed through a bath of theslurry, excess of which was removed from the surface of the impregnatedweb by feeding it through the gap (0.7 mm wide) between two horizontallydisposed steel bars of diameter 19 mm. (Passage between the nip of apair of rollers is an effective alternative.)The impregnated tissue wasthen dried by passage through a hot-air dryer at 100° C.

The flexible sheet material thus obtained had mass/unit area 710grams/m², thickness 0.7 mm, and compression (ASTM F806) 35% under astress of 34.5 MPa.

The material was applied to each side of a tanged steel support sheetcoated with heat-activatable adhesive, and the resulting sandwich wasfed between heated rollers. Automotive exhaust gaskets of the desiredshape and aperture were then stamped from the composite sheet thusformed.

EXAMPLE 2

Following generally the procedure of Example 1 glass web of weight perunit area 102 grams/m² was impregnated with a slurry having thefollowing composition:

    ______________________________________                                                      % by dry weight                                                 ______________________________________                                        Calcined china clay                                                                           83.1                                                          Colloidal silica                                                                              5.6                                                           Acrylic latex   11.3                                                          ______________________________________                                    

Sheets of the glass web were soaked in a bath of the slurry and passedbetween two bars having a separation of 0.78 mm. The impregnated sheetswere then dried and found to have the following properties:

    ______________________________________                                        Thickness              0.78 mm                                                Mass/unit area         765 grams/m.sup.2                                      Compression at 34.5 MPa                                                                              32.3%                                                  Cross-grain tensile strength                                                                         12.8 MPa                                               ______________________________________                                    

The tensile strength of the flexible sheet material was also measuredfollowing exposure to high temperature for a period of 5 hours, and thefollowing results obtained:

    ______________________________________                                        Test temperature                                                                            Cross-grain tensile strength                                    ______________________________________                                        300° C.                                                                              9.3 MPa                                                         500° C.                                                                              4.4 MPa                                                         700° C.                                                                              2.8 MPa                                                         ______________________________________                                    

The tensile strength of a conventional cellulose-based paper gasketmaterial was not measurable in the above tests because the material wastoo fragile to be handled.

In addition the stress relaxation behavior of the flexible sheetmaterial was measured by a similar method to that described in ASTMF1278, using an annular specimen of internal diameter 14.7 mm andexternal diameter 34.5 mm. The test rigs were machined from anickel-chromium alloy (Nimonic) to allow the test to be performed athigh temperature. An initial stress of 20 MPa was applied to thespecimen and the residual stress was measured after exposure to atemperature of 500° C. for periods of 16 and 69 hours. The loss instress as a percentage of the .initial stress was 49% in each case. Bycontrast, a conventional cellulose-based paper gasket material had ahigh stress relaxation (91%).

EXAMPLE 3

Following generally the procedure of Example 1 glass web of weight perunit area 70 grams/m² was impregnated with a slurry having the followingcomposition:

    ______________________________________                                                      % by dry weight                                                 ______________________________________                                        Calcined china clay                                                                           88.8                                                          Colloidal silica                                                                              5.6                                                           Acrylic latex   5.6                                                           ______________________________________                                    

Addition of sodium carbonate was omitted from the mixing procedure, toform a lower-viscosity slurry which did not have the thixotropicproperties of that in Example 1. Satisfactory impregnation was stillachhieved, and after drying the impregnated web was passed through thenip of a calender. The resulting sheet material had the followingproperties

    ______________________________________                                        Thickness             0.62 mm                                                 Mass/unit area        612 grams/m.sup.2                                       Compression at 34.5 MPa                                                                             26.7%                                                   Stress relaxation     38% (16 hours                                                                 at 500° C.)                                      ______________________________________                                    

In the above Examples, impregnation of the non-woven web of inorganicfibres is carried out by immersing the web in the aqueous slurry ofinorganic filler. Other methods of impregnation can be employed, forexample: application of the slurry to the web in air, followed byremoval of excess slurry, as in knife-over-roller coating.

We claim:
 1. A method of making non-asbestos flexible sheet material foruse in gaskets operating at temperatures of 500°-700° C. comprisingproviding a non-woven web of fibers which consist of an inorganic,non-asbestos material stable at 700° C., impregnating the web with anaqueous slurry consisting of inorganic filler and a mixture ofelastomeric binder and inorganic binder; and then drying the impregnatedweb.
 2. A method according to claim 1, in which the non-woven web is ofglass fiber.
 3. A method according to claim 1 in which the inorganicfiller is a clay.
 4. A method according to claim 3, in which the clay isselected from the group consisting of ball clay, china clay and calcinedchina clay,
 5. A method according to claim 1 in which the elastomericbinder is an acrylic elastomer.
 6. A method according to claim 1 inwhich the elastomeric binder is a nitrile butadiene rubber.
 7. A methodaccording to claim 1 in which the inorganic binder is a silica binder.8. A method according to claim 7, in which said inorganic binder is inthe form of colloidal silica.
 9. A method of making non-asbestosflexible sheet material suitable for use in gaskets operating attemperatures of 500°-700° C., in which a non-woven web of inorganicnon-asbestos fibers stable at 700° C. is impregnated with an aqueousslurry consisting of 75-95 dry wt % of inorganic filler, from 1-12 drywt % of a dispersed elastomeric binder and from 1-15 dry wt % of adispersed or dissolved inorganic binder, and then drying the impregnatedweb.
 10. Non-asbestos flexible sheet material of inorganic fibers madein accordance with claim
 1. 11. An automobile exhaust manifold gasketcomprising a metal support and, bonded to it, a facing of non-asbestosflexible sheet material of inorganic fibers according to claim 10.